1. Field of the Invention
The present invention relates generally to controls rods of a nuclear reactor. More specifically, the present invention relates to a control rod latch tool which facilitates the release and removal of the control rods when replacement is required.
2. Description of the Relevant Art
FIG. 1 is a perspective partially cut-away view of a boiling water reactor of the type to which the present invention is applicable. This BWR (boiling water reactor) includes, merely by way of example, a head 40 which is secured to the top of the reactor vessel 41, a vent and head spray 42, a steam dryer lifting lug 43, a steam drier assembly 44, a steam outlet 45, a steam separator assembly 46, a feedwater inlet 47, a core spray inlet 48, a feedwater sparger 49, a low pressure coolant injection inlet 50, a core spray line 51, a core sprayer sparger 52, a top guide 53, a jet pump assembly 54, a core shroud 55, fuel assemblies 56, a jet pump/recirculation water inlet 57, a core plate 58, control rods 59 (enclosed in elongate tubular guide thimbles), a recirculation water outlet 60, a vessel support skirt 61, a shield wall 62, control rod drive mechanisms 63, control rod drive hydraulic lines 64, and an in-core flux monitor 65.
In this type of BWR the control rods 59 have an essentially cruciform cross-section, and, in order to facilitate refueling (viz., replacement of the fuel assemblies), are disposed at the bottom of the reactor vessel in a manner wherein they can be selectively moved up between the fuel rod assemblies by the hydraulically operated locking piston type control rod drive mechanisms 63.
FIG. 2 shows an example of a control rod 59 which is used in this type of reactor. This illustrated arrangement includes a lifting handle 66, upper guide rollers 67, a sheath 68, a plurality of neutron absorbing rods 69 enclosed in the sheath to define four blades 70, a coupling release or unlatching handle 71, a velocity limiter 72, lower guide rollers 73, and a coupling socket 74.
Details of the construction of the lower portion of a fuel rod is shown in FIGS. 3 and 4. As shown, the coupling socket 74 houses a lock plug 75 and a lock plug return spring 76 which are arranged at the lower end of an actuating shaft 77. When coupled to a control rod drive, the coupling socket 74 is arranged to operatively engage with a spud 78, and an unlocking tube 79 which extend up from an index tube--drive 80, in the manner illustrated in FIG. 4.
In order to disconnect a control rod 59 of the above described nature from an associated control rod drive, it is necessary to raise the unlatching handle 71. This releases the coupling arrangement shown in FIG. 4 and allows the control rod 59 to be lifted by the lifting handle 66 located at the top of the rod, up and out of the reactor vessel 41.
As is well known, when a reactor of the illustrated type is opened for refueling or maintenance, the vessel 41 is flooded with water in order to suppress radioactive emissions. This, in combination with the fact that the control rods 59 are located deep in the reactor vessel 41, as will be appreciated from FIG. 1, renders access very difficult. Nevertheless, it is required to be able to periodically remove the control rods and/or the control rod drives from the reactor vessel.
FIGS. 5 to 10 show an existing latch tool which is used to lift the unlatching handle and release the control rod 59 from its respective control rod drive.
This tool basically comprises a stud 81 which extends upward from a frame 82 for connection to a crane line, a tube assembly 83, an actuator link air cylinder 84, a lifting hook air cylinder 85, and an actuator link 86 which is adapted to engage the unlatching handle 71. The actuator link 86 is operatively connected with the air cylinder 84 by way of an actuator rod 87. As shown, the actuator link 86 is supported near the bottom of a V-cross-section structural member 88.
Details of a lifting hook structure 89 and the manner in which it is pivotally mounted and connected with the lifting hook air cylinder 85 are illustrated in FIG. 6.
In order that the actuator link 86 be accurately located, the arrangement is provided with a short blade-shaped back guide member 90. Accordingly, when this tool 100 is put into use, the tool engages the unlatching handle of the control rod 59 from one side while the back guide 90 provides an aligning function on the other. The actuator link air cylinder 84 and the lifting hook cylinder 85 are supplied with air under pressure by way of air connectors 91.
However, with this existing type of arrangement it is necessary for the fuel support piece to be removed before suitable disposition and alignment of the tool can be achieved. This removal is quite time consuming. Accordingly, it has been proposed to remove the back guide 90, from the latch tool in order enable its use without the need to remove the fuel support piece. However, this measure has not met with success in that, without the back guide tool engagement is unreliable and results in problems with the engagement to the unlatching handle.
An alternate technique which has been proposed in order to save time and avoid the handling problems related with the removal of the fuel support piece, is such as to use a J-hook which is suspended from a cable, to unlatch the control rod 59.
However, this technique suffers from the drawbacks that, as the unlatching handle is submerged in approximately 60 feet of water, in order for the operator to be able to observe the hook and control the maneuvering of the same, it is necessary to utilize a TV camera and immerse it to essentially the same depth as the hook.
Correct positioning of both the hook and the camera is time consuming, requires at least two operators and requires a certain amount of experience and skill.